Incandescent lamp



Patented Sept. 14, 1926.

UNITED STATES 1,600,203 PATENT OFFICE.

PERCY A. CAMPBELL, OF NEWARK, NEW JERSEY, ASSIGNOB TO GENERAL ELECTRIC COMPANY, A CORPORATION OF NEW YORK.

INCANDESCENT LAMP.

No Drawing.

The present invention relates to incandescent lamps. Itsobject is to improve the operation of these lamps and particularly to obviate tliediscoloration of the bulb which sometimes occurs early in the life of'the lamp, that is, before the deposition of a film by the gradual accumulation of material evaporated from the incandescent filament during the normal operation of the lamp.

This premature discolor-ation of the bulb of tungsten filament incandescent lamps is usually accompanied by a decrease in the useful life of the lamp. Apparently, the discoloration is an indication of the presence of Water vapor in the lamp. It is not liable to appear if the lamp is given long and careful exhaust at high temperature, but it is desirable that lamps exhausted by the commercial methods ordinarily used in the lamp factory operate Without discoloration and With a normal length of life.

In accordance with my invention I apply to the filament of the lamp some carbon preferably in such a form that it is uniformly distributed on the filament in a finely divided state. When the filament is incandesced the carbon disappears, but apparently by chemical combination with deleterious gases exerts some influence or effect which prevents early discoloration of the bulb and enables me to produce a lamp which will 0perate for a normal useful life without objectionable blackening.

I prefer to use carbon in the very finely divided form found in the commercial product known as Aquadag which I understand contains graphite in a colloidal form. The amount of carbon necessary to produce the desired results is very small, ordinarily of the order of about of one per cent of the weight of the filament. Too much carbon does not discolor the bulb but weakens the filament apparently by chemically combining therewith.

The finely divided carbon may be applied to the filament in various ways as long as it is distributed with substantial uniformity. A convenient method which I have used with success is to add to commercial concentrated Aquadag enough distilled water to produce a liquid containing above five per cent by Weight of concentrated Aquadag in suspension. This liquid may be applied upon the filament in a very fine spray obtained by the use of an atomizer or spraying bottle. If

Application filed March 2, 1922. Serial No. 540,611.

the filament is heated to a temperature about that at which paper scorches, the carbon in suspension will stick to it. A convenient way of applying the carbon is to distribute the filaments uniformly over a sheet of paper on a rotating -llOt plate. The temperature can be raised to the point where the paper begins to'scorrh, and when this occurs, the liquid containing five per cent Aquadag is sprayed over the paper and the filaments as uniformly as possible. If the spraying is conducted under proper conditions, the carbon will sticlrto the filaments and dry on them as fast as sprayed, but if the temperature is not high enough, black streaks will be found on the paper beneath the filaments.

I have to nail that the right amount of car bon will be applied to the filaments if about 20 cubic centimeters of the liquid containing 5% of Aquadag is uniformly sprayed over an area of about 70 to 80 sq. inches. The filaments should be distributed over this area so that every filament will be exposed as much as possible to the spray.

For most purposes it is advantageous to mix some red phosphorus with the carbon. I have obtained good results with a suspension in water of about equal parts by weight of carbon and of red phosphorus. To obtain a suitable mixture of phosphorus and carbon, I prefer to suspend in distilled Water enough finely ground red phosphorus to make a liquid containing about 3% by weight of phosphorus. I have found that when phosphorus is used less carbon is necessary, and that only about 10 cubic centimeters of the Aquadag solution is required for spraying an area of 70 to 80 sq. inches if about 5 cubic centimeters of this liquid containing 3% of phosphorus is mixed with Aquadag solution. This smaller amount of carbon is advantageous, as a comparatively large amount of carbon on the filament causes difiiculty in factory operations, particularly in welding the carbon coated filament to the leads of the lamp.

I prefer to mix the liquids containing the Aquadag and the phosphorus at the time of using them, as the carbon has a tendency to settle out if the mixture containing phosphorus and carbon stands for a long time. If too much phosphorus is added a discoloration of the bulb will be observed and if too much carbon is added, there will be an attack upon the filament.

The carbon may be applied in other ways than by spraying as for example, by dipping the filament into a suspension of the proper concentration, but it is more diflicult to obtain the proper distribution of the carbon by dipping the filament than by spraying it.

Forms of carbon besides Aquadag can be used in the practiceof my invention. I have obtained good results with inks such as india ink, in which finely divided carbon is suspended; and with finely divided lamp black. If desired, the carbon may be derived from compounds which decompose in contact with the heated filament and leave a residue. of carbon, as for example, starch, gum arabic and various aniline dyes.

After the filament is coated with carbon I it is mounted in the bulb and then either types.

during exhaust or after the lamp is sealed oil the filament is brought up to incandescence whereupon the carbon disappears from the filament and produces the desirable results above described, although the'presenee of the carbon in the lamp is difiicult to detect.

My invention is applicable to various types of lamps but I have obtained the best results withgas filled lamps having closely coiled tungsten filaments of the concentrated Gas filled lamps are described in Langmuir Patent l lo 1,180,159. The greatest difiiculty with discoloration and short life is encountered in lamps such as are used for headlights on automobiles and which are constructed to operate at voltage of 12 to 24 volts. In such lamps my invention can be used to particular advantage, although useful on many other types and sizes of lamps.

lVhat I claim as new and desire to secure by Letters Patent of the United States, is:

1. An incandescent lamp comprising the combination of a bulb, an incandescent body of non-carbonaceous refractor material therein, and a quantity of soli carbonaceous material applied to the surface of said body in a quantit effective to prevent discoloration of said iiulb during the early part of the life of said lamp but insufficient to materially change the physical properties of the body.

2. An incandescent lamp comprising the combination of a bulb, a filament of tungsten therein, and finely divided carbon applied to the surface of'said filament in an amount of about one-tenth per cent of the weight of the filament.

3. An electric lamp comprising a bulb, a tungsten filament therein, a filling of gas therein which is inert with respect to said filament and a quantity of carbon applied to said filament, said carbon being of such small amountms to be dissipated by incandescenee of the filament but large enough 0 1prevent premature discoloration of said 4. An article of manufacture comprising a lighting body of refractory metal and a quantity of carbon applied to the surface of said body in a quantity suificient to revent premature discoloration of a lamp ulb in which the lighting body is operated but insufiicient to materially change the physical properties of the body.

5. An article of manufacture comprising a lighting body of tungsten wire and a coating of carbon provided on the surface of said wire in an amount of about one-tenth per cent of the weight of the tungsten wire.

In -witness whereof I have hereunto set my hand this 24th day of February, 1922.

PERCY A. CAMPBELL. 

